Foundry sand for molding has been mixed and adjusted by adding an additive such as bentonite and water to reclaimed sand. In such a mixing operation, the water content value of the mixed sand greatly affects the moldability of the mixed sand. Therefore, a skilled worker adjusts the water content manually by feel, for example by grasping the mixed sand. In an automatic machine, because there is an extremely meaningful correlation between the water content value of the mixed sand and the CB value (compactability value) of the foundry sand, the CB value is automatically measured to adjust the water content. When mixing (mulling) adjustment is performed in such an automatic machine, the shortage of water, that is, the difference between the actual and target water content of the mixed sand, is determined from the water content of sand charged into the sand muller, which is determined by a water sensor, and the lacking amount of water is supply in (primary supplied water). Then, the lacking CB value, that is, the difference between the actual CB value and target CB value, is determined by measuring the CB value during mixing, the water content corresponding to the lacking CB value is determined from the water content−CB value correlation factor, and the additional amount of water corresponding to the lacking water content is supply (secondary supplied water). The correction amount of the primary supplied water is then determined from the state of the past additional pouring of water, and the correction amount is reflected as feedback to the amount of the primary supplied water in the next mixing cycle. The above-described technique is disclosed in Japanese Patent Application Publication No. S61-14044.